I was perhaps unclear. I meant a firmware blob in the sense of an OS image that includes Linux, such as might be downloaded as an update to your smartphone.

Is a disk image that includes Linux and some proprietary software sketchy?

I don't really see the difference-- as long as you have tools to pull apart the disk image.

Can you delete the proprietary software, and if so does the Linux still run?

Can you modify the Linux part, and if so does the proprietary software still run?

If those two criteria are satisfied I think you could probably successfully argue that they are cleanly separable components within a "mere aggregate". If not, they pretty clearly are bound together into a larger combined work.

> Can you delete the proprietary software, and if so does the Linux still run?

Generally, yes. Of course, some people have a custom init, and I don't think this really breaks things.

> Can you modify the Linux part, and if so does the proprietary software still run?

Yup, generally. Of course, the device itself may prohibit you from loading arbitrary software on it without rooting it or modifying the bootloader. You may not have a license to run the proprietary software with modified Linux, though.

You mention android smartphones above. The existence of LineageOS, etc, is evidence enough that you can do these things.

Generally, if you modify either the Linux part OR the proprietary part on a smartphone, the machine will fail to boot (i.e. the software will not run). This is a deliberate design goal, cryptographically enforced, and it is not implemented in physical hardware, but in the very software in question - the boot process includes some sort of "integrity check". This is the clearest possible statement of intent to integrate the two into a single combined work.

LineageOS proves nothing. It's a separate work.

I am not 100%, but inclusion of GPL software is not prohibited in non-GPL works as long as the GPL work is made available, at least upon request.

It boils down to the distinction between "derived work" and "mere aggregate", which is currently legally vague. This vagueness has allowed all sorts of shenanigans that violate the spirit of the GPL.